Re: [Haskell-cafe] Re: Why is $ right associative instead ofleftassociative?
On Feb 8, 2006, at 1:34 AM, Stefan Monnier wrote: The trouble with monad comprehensions was that it became far too easy to write ambiguous programs, even when you thought you were just working with lists. One solution was already suggested: to make the comprehension syntax be pure syntactic sugar whose semantics depends on the semantics of the identifiers the syntactic sugar expands into. OK. Which identifiers? I happen to want a version which always uses "concatMap" (or, equivalently, monadic bind), and never, ever the direct "efficient" translation. To get the efficient translation for lists a la Wadler, though, this requires either a wrapper, so that the comprehension runs at the type ([a] -> [a]) and gets applied to [] at the very end, or it requires heavy lifting from the compiler (foldr/build and its kin as seen in GHC, phc, etc.). When it was all tied to lists, it was easy to gloss over the details of the machinery. -Jan-Willem Maessen So you could keep the current list-only comprehension as default, and allow monad comprehension by providing a library (which the users need to import so as to hide the Prelude's definition). Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Why is $ right associative instead ofleftassociative?
> The trouble with monad comprehensions was that it became far too easy to > write ambiguous programs, even when you thought you were just working with > lists. One solution was already suggested: to make the comprehension syntax be pure syntactic sugar whose semantics depends on the semantics of the identifiers the syntactic sugar expands into. So you could keep the current list-only comprehension as default, and allow monad comprehension by providing a library (which the users need to import so as to hide the Prelude's definition). Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why is $ right associative instead ofleftassociative?
On Sun, Feb 05, 2006 at 06:50:57PM +, Ben Rudiak-Gould wrote: > Paul Hudak wrote: > >Minor point, perhaps, but I should mention that : is not special syntax > >-- it is a perfectly valid infix constructor. > > But Haskell 98 does treat it specially: you can't import Prelude hiding > ((:)), or rebind it locally, or refer to it as Prelude.:. In fact I've > always wondered why it was done this way. Can anyone enlighten me? Of > course it might be confusing if it were rebound locally, but no more > confusing than the fact that [f x | x <- xs] is not the same as (map f xs). > > It might be kind of nice if the list type were actually defined in the > Prelude as > > data List a = Nil | a : List a > > and all of the special [] syntax defined by a desugaring to this (entirely > ordinary) datatype, e.g. [1,2] -> 1 Prelude.: 2 Prelude.: Prelude.Nil. it would probably be simpler just to declare [] to be a data constructor. that is what jhc does, it parses the same as any capitalized name. so you can do import Prelude hiding([]) data Foo a = [] | Foo | Bar and list syntax desugars into whatever (:) and [] are in scope. similarly, (x,y) is just sugar for (,) x y and (,) is a standard data constructor and can be hidden, redefined, etc just like any other one. John -- John Meacham - ⑆repetae.net⑆john⑈ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why is $ right associative instead ofleftassociative?
Ben Rudiak-Gould wrote: Paul Hudak wrote: Minor point, perhaps, but I should mention that : is not special syntax -- it is a perfectly valid infix constructor. But Haskell 98 does treat it specially: you can't import Prelude hiding ((:)), or rebind it locally, or refer to it as Prelude.:. In fact I've always wondered why it was done this way. Can anyone enlighten me? I think that originally it was because various primitives were defined (via "Translations" in the Haskell Report) in terms of lists. But with qualified imports I'm also not sure why this is necessary. Of course it might be confusing if it were rebound locally, but no more confusing than the fact that [f x | x <- xs] is not the same as (map f xs). It's not? Hmmm... why not? (At one time list comprehensions were another way to write do notation -- i.e. they were both syntactic sugar for monads -- in which case these would surely be different, but that's not the case in Haskell 98, as far as I know.) -Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Why is $ right associative instead ofleftassociative?
Paul Hudak wrote: Minor point, perhaps, but I should mention that : is not special syntax -- it is a perfectly valid infix constructor. But Haskell 98 does treat it specially: you can't import Prelude hiding ((:)), or rebind it locally, or refer to it as Prelude.:. In fact I've always wondered why it was done this way. Can anyone enlighten me? Of course it might be confusing if it were rebound locally, but no more confusing than the fact that [f x | x <- xs] is not the same as (map f xs). It might be kind of nice if the list type were actually defined in the Prelude as data List a = Nil | a : List a and all of the special [] syntax defined by a desugaring to this (entirely ordinary) datatype, e.g. [1,2] -> 1 Prelude.: 2 Prelude.: Prelude.Nil. -- Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
